Corrosion poses significant challenges to metals used in construction and machinery. This study evaluated the inhibitive effect of papaya (Carica papaya) leaf extract, obtained through Ultrasound–Microwave-Assisted Extraction (UMAE), as an eco-friendly corrosion inhibitor for mild steel. Papaya (Carica papaya) leaves are rich in phytochemicals such as alkaloids, flavonoids, tannins, and saponins, which form protective layers on metal surfaces and reduce degradation. UMAE, combining ultrasonic and microwave-assisted extraction, proved more efficient than conventional Soxhlet extraction, yielding 63.43% compared to 6.78%, while preserving bioactive compounds. Papaya (Carica papaya) leaf extract (PLE)-coated mild steel (MS) strips were immersed in hydrochloric acid (HCl) solutions (0.5 M, 1 M, 1.5 M) and monitored over various exposure times. Weight loss, corrosion rates, and inhibition efficiency were measured, with the lowest weight loss of 2.1798 g under four coatings of PLE after 336 hours and a peak efficiency of 97.63%. Characterization techniques such as SEM-EDX, FTIR, and UV-Vis confirmed the formation of corrosion-resistant barriers and identified key functional groups responsible for the PLE’s protective properties. Statistical analysis using ANOVA revealed that PLE has a significant relationship with inhibition efficiency, with an F-value of 13.69035, p-value of 0.001638, and F crit less than the F-value. These findings position PLE as a promising, sustainable solution for corrosion prevention in industrial applications.